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How consciousness is represented in the brain?

Humans have the ability to learn and interact with the world being aware of their own experience.  This knowledge about one´s awareness is called “metacognition” and it is one of the major characteristics that differentiate humans from other species. But, what are the brain mechanisms underlying consciousness?  How does the brain create different consciousness states?  Modern Neuroscience has investigated these questions for centuries, but yet we have no idea how consciousness arises in the brain. Although brain technology has hugely progressed in the last decades, for researchers is very difficult to study conscious experiences because they are entirely subjective and cannot be accessed by others. Thus, measuring consciousness in experimental conditions it is almost impracticable without getting contaminated by subjective biases.

Thanks to language – another singular human ability not shared with other species- we are able to communicate our conscious experience to others and to asses the subjective experiences of other individuals. For example, we can both be watching the same film but I have no way to tell whether my experience of watching that movie in my screen is different or similar to yours unless you tell me about it. If you were not able to communicate in any of the language systems not even with signs,  I would not have a way to tell whether you’re conscious or not.

In a way, language gives us a strong proof of our consciousness but it does not tell us how arises and how it is represented in the brain.  A new study recently published in Science Advances has identified for the first time the brain patterns of consciousness.  But let’s understand better what this study has discovered so far.

Brain connections linked to consciousness

We know that prefrontal cortex is involved in a number of cognitive functions including conscious thinking. However, the prefrontal cortex is a large area that connects with other brain regions creating many different networks. A way of determining how consciousness is represented in the brain is by differentiating the network properties specifically associated with conscious from those associated with unconscious states. Some of these network properties that can be studied to understand consciousness are the modularity, neural integration, distance relationship, and efficiency.

Using functional magnetic resonance imaging (fMRI), researchers of the referred study measured the activity of the brain and the patterns of neural “communication”  linked to different conscious states in 47 healthy individuals and 102 brain injury patients with reduced (59) or absent (53) conscious activity. The reported findings show two main patterns of communication across regions. One simply reflected structural connections between regions of the brain that have a direct physical link. This pattern was seen in unresponsive patients with no conscious experience. A second pattern represented much more complex brain-wide interactions across 42 brain regions belonging to six brain networks all with important roles in cognition. Importantly, this complex pattern was only present in people with some level of consciousness and disappeared when patients were under deep anesthesia.

brain maps

The figure shows the brain connectivity differences in consciousness and unconsciousness.  Red colors indicate positive connections and blue indicate negative connections. The brain pattern of conscious individuals exchanges information showing both positive and negative connections while unconscious individuals show a pattern of brain configurations that connects less with other regions. Image taken from  E. Tagliazucchi & A. Demertz

The results of this study indicate that brain organization during conscious wakefulness differs from the unconscious states. Brain signatures of conscious states rest on patterns of complex neural activity with signals fluctuating between states of high and low connectivity in long-distance brain regions. In contrast,  loss of consciousness is characterized by a more sustained pattern of low interregional connectivity.

In light of these exciting findings, we have come closer in our understanding of how consciousness emerges in the brain which brings important implications. Now, neurologists and clinicians can identify the brain signatures that indicate consciousness without relying on self-report or the need to ask patients to engage in a particular task. Also, neurosurgeons will be able to differentiate between conscious and unconscious patients after brain injury.  Still,  other questions arise that need to be investigated in the future such as whether the presence of these patterns in people with minimal conscious entails the presence or absence of cognitive activity or mental contents (e.g. thoughts).

ABOUT THIS RESEARCH ARTICLE

Original Research: Open access research for “Human consciousness is supported by dynamic complex patterns of brain signal coordination” by A. Demertzi, E. Tagliazucchi, S. Dehaene, G. Deco, P. Barttfeld, F. Raimondo, C. Martial1, D. Fernández-Espejo, B. Rohaut, H. U. Voss, N. D. Schiff, A. M. Owen, S. Laureys, L. Naccache and J. D. Sitt in Science Advances. Published February 1 2019.
doi:10.1126/sciadv.aat7603

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